Creating a Fluffy Cloud
Here's what your file looks like rendered with the Sun lamp strength set to 4 (it was set to 1):



The key to having a good "fluff" is a wide dynamic range. Dynamic range is the distance between the brightest and darkest parts of an image. See Render with a wider dynamic range in cycles to produce photorealistic looking images for a good introduction to a whole rabbit hole of lighting, grading, and generally managing the dynamic range of your scene and image.

The simple answer

For the simple answer, suffice it to say that the sun is bright. By making the sun lamp significantly brighter than other light sources, you'll start to get an increased dynamic range and clouds will take on more of a "fluffy" look (bright highlight falling off to shadow) when seen from the side.

By using a bright sun lamp and messing with camera settings (exposure, contrast etc.) to get the final image to have the overall brightness you want, you can get a decent physically-based-ish result without too much fuss.

The proper answer

We know that the sun lamp energy in Blender is measured in terms of irradiance, or the amount of energy (light) from the source hitting another object at a given point in time (the exact units are Watts / meters²).

The Sun's irradiance measured on Earth varies over a whole host of things, not least of which is how much atmosphere the sunlight must go through. Assuming we are simulating light scattered by the atmosphere separately (e.g. with an emissive sky texture), we'll want to make the sun strength something like 1050 W/m². If instead we are sparing no expense and simulating the atmosphere with a large volumetrically scattering object, we'll want to include atmospherically scattered light in the sun lamp and make it more like 1361 W/m² (taking values from Wikipedia).

If this seems insanely bright, that's because it is (have I mentioned the Sun is bright?). By using Sun lamp values around the default of 1, we're putting our Blender scenes over 4,300,000,000 km farther away from the Sun than the Earth (somewhere beyond Neptune)!

Here's an example with a 1050 W/m² Sun lamp, a 300 W/m² sky, and a camera exposure of -7 (with filmic):